Pulsed nuclear magnetic resonance (NMR) measurements alternate between transmitting high-powered radio-frequency (r.f.) pulses and receiving low-level response signals in a matter of a few ten or hundred microseconds. The combination of a strong static magnetic field and radio frequency pulses tend to excite mechanical resonances within the measurement apparatus, which resonances in turn cause an interference signal induced in the receiver system by a microphonic effect.
It has long been known that the interference arising from imperfect "refocusing" pulses can be canceled by repeating the measurement with the r.f. phase of the refocusing pulses inverted. This phase reversal does not affect the NMR signal, but inverts the phase of the interference. By acquiring both magnitude and phase of the compromised signals and by adding complex-valued measurements, the NMR signal is enhanced, while the "refocusing" interference is eliminated.
The above error cancellation scheme has become standard in practice, but it does not address interference problems arising from the "excitation" pulse, which typically is the first pulse in a long series of pulses. Changing the excitation phase would also change the phase of the NMR signal: excitation interference and NMR signal are always in phase with each other. Since often only the first data point ("echo") is affected by excitation interference, it is customary to eliminate this first data point from the data set. The first data point, however, contains valuable information about fast time-dependent behavior of the NMR sample and therefore having to ignore this point is an unsatisfactory solution.
The method of the present invention, described in more detail below, uses a novel cycle of pulse sequences to reduce the effect of "excitation" interference, on the basis of changing the measurement frequency between certain pulse sequences. Naturally, the method is especially useful for NMR measurements in which small changes in frequency can readily be allowed or tolerated. For example, laboratory-type NMR machines typically operate in homogeneous fields with a single, well-defined frequency. Changes in frequency are employed either to follow fluctuations in the main magnetic field, or to enable magnetic resonance imaging (MRI). NMR machines built for wireline logging or similar industrial applications are much more robust with respect to small changes in frequency. Therefore, the proposed solution is well-suited for industrial NMR applications.
The method of the present invention uses prior art NMR apparatuses and logging tools to obtain previously unavailable data relating to the fast time-dependent behavior of an NMR sample. In particular, a novel pulse sequence is proposed and used to obtain improved NMR data by eliminating spurious signals corresponding to mechanical resonances in the measurement apparatus induced by the r.f. excitation pulse.